The present invention relates to a speaker system having a bass-reflex cabinet.
In the speaker system, the formation of the cabinet affects the reproduction of low frequency sound. There are various types of the cabinets, such as an air-tight cabinet and a bass-reflex cabinet. The air-tight cabinet comprises a closed box for absorbing the sound wave radiated from a speaker unit to at the rear portion of the cabinet. The bass-reflex cabinet is provided with a port formed in the front panel of the closed box. The sound rearwardly radiated in the cabinet is discharged from the port for increasing the low frequency characteristic.
FIG. 10 shows a conventional bass-reflex cabinet. A cabinet 1 comprises a speaker unit 2 secured to the front panel of the cabinet 1 in a central portion thereof. A bass-reflex duct 3 having a predetermined length is secured to the front panel of the cabinet 1 at a lower portion of the cabinet 1. A sound absorbing material 4 is laid on the inside walls of the cabinet 1.
When the speaker unit 2 emits sound, a part of the sound is radiated in the rearward direction in the cabinet 1 and enters the duct 3 from an opening 3b. The sound is corrected into the same phase as the sound forwardly radiated from the speaker unit 2 and emitted from a port 3a. Thus, the low sound characteristic is increased.
Since the duct 3 is opened at both ends, impedance as view from one end is extremely reduced when EQU fn=n(c/2L'),
because of resonance,
where fn is the frequency, c is the sound velocity, L' is an equivalent length of the duct, and n is an integer. The equivalent length L' is determined by the length L of the duct and a correcting value .DELTA.D of the diameter D of the port of the duct.
The frequency fn is generally in a range of 300 Hz or more which is a middle sound range. Namely, the middle sound range is enhance, so that the low sound is not increased. In addition, the sound pressure in the cabinet is reduced due to the resonance. Consequently, the sound pressure of the sound forwardly emitted from the speaker reduces, so that the frequency characteristic is uneven and the transient characteristic is also deteriorated.
In the bass-reflex cabinet, the low frequency characteristic is adjusted by determining the opening area and the length of the duct. In order to obtain a desired low frequency characteristic, in the case that it is necessary to use the duct having a long length, a long duct must be bent in the cabinet. However, velocity of the sound wave in the bent duct is not constant at a bent portion thereof. Since the velocity becomes faster at an inner corner of the bent portion to increase the resistance, so that the resonance Q is reduced. Accordingly, the low frequency characteristic reduces.
Furthermore, the duct 3 is cantilevered on a front panel of the cabinet 1. Consequently, the duct 3 may be vibrated by the sound pressure during the reproduction. The vibration generates a sound pressure which cause the quality of the reproduced sound to be deteriorated.